1. Field of the Invention
The present invention relates to a prothesis adapted to be fixedly attached to bone by a cement. More particularly, the present invention is directed towards a prothesis adapted to maximize the strength and durability of the prothesis/bone cement adherence.
2. Description of the Prior Art
In the field of orthopedic surgery, ZIMALOY manufactured by Zimmer, U.S.A. Inc., a chromium-cobalt-molybdenum alloy, stainless steel, titanium alloys, and polymerized materials such as ultra high molecular weight polyethylene (hereinafter UHMWPE) have been used successfully to replace the ends of long bones and joints, including the hip joint. However, there exists a severe limitation with respect to such orthopedic surgery, namely, coupling of the prosthesis to bone. Due to such factors as mechanical stress, fatigue, corrosion, etc., the prosthesis/bone cement joints have been prone to failure.
Present methods of utilizing such bone prosthesis involve the use of a prosthesis having a stem portion which is inserted into the interior of a bone. A bone cement comprising a mixture of polymethylmethacrylate (hereinafter PMMA) polymer and methyl methacrylate monomer and optionally including a styrene copolymer of PMMA is likewise inserted into the bone cavity and utilized to couple the stem of the implant to the bone itself. Experience has demonstrated, however, that serious drawbacks exist with respect to the coupling between the prosthesis stem and the bone cement. Attempted solutions to this problem have been directed primarily toward strengthening the prosthesis/bone cement interface by means of gross mechanical interlock involving, for example, dove tails, small stems, and the like. Such devices result in stress concentrations that can exceed the strength of the bone cement as well as cause non-physiological force distribution in the bone.
Adherence at the interface between the implant and PMMA is greatly restricted by current industrial and surgical practices. For instance, the PMMA cement is typically applied in a highly viscous, doughy state with the result that the degree of contact between the implant and the cement is inadequate. Moreover, the existence of wear boundary layers such as contaminants and weak metal oxides on the surface of the implant have also caused problems. Weak boundary layers may be due to the composition of the implant or to the process of forming the same. Thus, in the case of a metal implant, the surface of the implant normally includes weak metal oxides as weak boundary layers. In the case of a polymeric implant, the surface of the implant normally includes a weak boundary layer comprising monomer, partially polymerized or low molecular weight polymer and contaminants comprising mold release agents, and the like. Finally, the implant may come in contact with air, blood or water prior to being inserted into the bone, thereby becoming contaminated. The existence of weak boundary layers, e.g., surface contaminants, is detrimental to the formation of good implant bone cement adherence. Thus, the strength of such joints has been dependent upon gross mechanical interlock. Such difficulties in the formation of a satisfactory prosthesis/bone cement connection have also caused the result that mere resurfacing of a deteriorated joint, e.g., a deteriorated hip joint due to arthritis, was not readily accomplished. Thus, in the case of a deteriorated articular surface, e.g., surface of the head or ball in a ball and socket joint, the entire head of the bone is generally removed and prosthetic head connected to the bone, although in some instances, resurfacing implants have been used with bone cement.
U.S. Pat. No. 4,336,618 to Simon Raab which is assigned to the assignee hereof, all of the contents of which are incorporated herein by reference, taught that the aforementioned prosthesis fixation problems could be overcome by treating at least that portion of the prosthesis which is adapted to be connected to bone in order to provide a PMMA film fixedly adhered to said portions of the prosthesis. Prior to the application of the PMMA film, the surface to be coated is treated to prevent formation of a weak boundary layer upon bonding of a bone cement to an applied film. Thereafter, a PMMA film is applied by dipping, painting, spraying, etc., and finally, after the film has dried, it is annealed to remove any stresses in the film.
The resultant prosthesis has a film of PMMA firmly adhered to the surface thereof. This PMMA film adhesively interacts molecularly with PMMA bone cement. Accordingly, the adherence of a prosthesis adhesively connected to bone by means of a PMMA cement can be drastically increased.
To summarize the teaching of U.S. Pat. No. 4,336,618 discussed hereinabove, the methods used for the application of the precoats typically include the preparation of the metal surface by cleaning and passivation, and then either solvent based lacquer polymerizing solutions of monomer catalyst, or inhibitor and polymer electrostatically-applied or dip-applied power coatings. In all cases, some curing and/or annealing heat cycles are used. The overall composition of the coating have been limited to PMMA and other standard approved inhibitors and catalysts.
Although the U.S. Pat. No. 4,336,618 is a great improvement over the previous prior art, the literature or implant fixation failure has focussed on the interface and bulk material strength measured typically in low strain/load rate tests. The strain and/or load rates can affect the nature of plastic failure modes. This can be best understood when one observes the fact that water becomes a very hard surface as in the example of a water skier landing in the water at a high speed. This impact in the water at high speed causes the water surface to become very hard. This same principle applies to plastics which are viscoelastic materials. The properties of viscoelastic materials change with the load application rate and the related strain or deformation rate.
Impact is defined as the application of a load over a short period of time such as 1/100th to 1 second. A plastic sample subjected to an applied load of 100 pounds will respond very differently if the period of time is ten seconds as compared to a period of time of one second. Plastics under impact load will deform far less before failure than under low rate loading. Additionally, plastic failure usually starts after elongation is finished and is generally due to fracture initiated at a flaw. The lack of strain or elongation during impact means that the fracture point is quickly reached and the material fails at a far smaller strain level than usual and/or is less able to absorb impact energy.
In lieu of the above and despite the large improvement in the prior art as taught by the U.S. Pat. No. 4,336,618, there appears to be a need for an improved bone connective prosthesis and method of forming the same to lower implant fixation failure rates and particularly those failures that are a result of impact and shock conditions.